Wireless communication systems are known in which mobile units (e.g., in-car mobile or in-hand portable radios) wirelessly communicate with a fixed communication infrastructure comprising a plurality of geographically-diverse transceivers. In such systems, methods for determining location information for a given mobile unit are known. In particular, the well-known weighted least squares (WLS) solution can be used to determine location information as shown, for example, in U.S. Pat. No. 5,416,712 issued to Geier et al.
Succinctly stated, the WLS approach to location determination attempts to iteratively derive a location estimate for a mobile unit based, in part, on distance estimates between the mobile unit and fixed transmitters having known locations. Given that distance can be calculated as the product of velocity and time, the distance estimates (referred to as pseudo-ranges or PRs) are calculated in practice by multiplying the propagation delays between the mobile unit and fixed transmitters with the speed of light. Assuming ideally measured propagation delays, the location of the mobile unit can be calculated using the pseudo-ranges with little or no error. However, propagation delays are measured in practice using transmitted signals, which signals are subject to the effects of various error sources, e.g., noise, multipath interference, distortion, etc. The resultant errors in the delay measurements are translated into errors in the pseudo-ranges and, consequently, into error in the location estimate. In order to combat the presence of measurement errors, the WLS solution factors the reliability of the various measurements into the location estimation.
Generally, location determinations in three-dimensional space require the reception of signals transmitted by at least four fixed transmitters. For example, in the well-known Global Positioning System (GPS), the receiving unit (i.e., the unit for which a location is to be determined) does not maintain a timing reference exactly synchronized to the highly stable and synchronized timing references maintained by the transmitting satellites. As a result, the pseudo-ranges determined by the receiving unit will not result in a precise location solution. In order to combat this, a fourth measurement is used to indicate exactly how far out of synch the receiving unit's local time base is with respect to the universal time base maintained by the satellites. In this manner, the receiving unit can determine a correction factor that, when applied to the pseudo-ranges, results in a more precise location solution. Additionally, by applying the correction factor to the local time base, the receiving unit can, at least for a period of time, serve as an accurate time reference relative to the universal time base.
This same concept may be readily applied to two-dimensional problems as well, e.g., land mobile radio communication systems where altitude measurements are of lesser importance. However, the same difficulty regarding synchronized time bases is also present in two-dimensional applications. It is known in the art to provide a common time base among fixed transceivers in a land mobile radio system. Indeed, such a common time base is often provided through the use of GPS receivers, described above. Additionally, as GPS receivers become increasingly less expensive, they may be more readily incorporated into mobile and portable units. Future developments may also provide for highly accurate time synchronization between fixed transmitters and mobile units. As such, a need exists for a method which incorporates the advantages of a common time base into a WLS location solution.